Synthesis and Biological Evaluation of Paclitaxel and Camptothecin Prodrugs on the Basis of 2-Nitroimidazole

Article abstract of DOI:10.1021/acsmedchemlett.7b00189

Due to the low esterase activity in human plasma, many ester and carbonate prodrugs tested in humans may be less effective than that in preclinical animals. In this letter, PTX and SN-38 were attached to the N-1 position of 2-nitroimidazole via a carbonate linker. Presumably, 2-aminoimidazole may help promote the intramolecular hydrolysis of the carbonate bond. The prodrugs exhibited a considerable stability in buffers at different pH values as well as in human plasma. Furthermore, a rapid reduction was exhibited in the presence of nitroreductase. An in vitro cytotoxicity assay demonstrated that hypoxic conditions could increase the toxicity of prodrugs. Potentially, the compound species may form a new class of promising antitumor agents.

Full text of DOI:10.1021/acsmedchemlett.7b00189

Letter
Synthesis and biological evaluation of paclitaxel and
camptothecin prodrugs on the basis of 2-nitroimidazole
JIN CHEN, Shuai Wen, Qiumeng Zhang, Qiwen Zhu, Jiahui Yu, and Wei Lu
ACS Med. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acsmedchemlett.7b00189 • Publication Date (Web): 27 Jun 2017
Downloaded from http://pubs.acs.org on June 28, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Medicinal Chemistry Letters is published by the American Chemical Society. 1155
Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 6
ACS Medicinal Chemistry Letters
1
2
3
4
5
6
Synthesis and biological evaluation of paclitaxel and camptothecin
prodrugs on the basis of 2-nitroimidazole
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a,‡
a,‡
a,‡
a
a
a
Jin Chen , Shuai Wen , Qiumeng Zhang , Qiwen Zhu , Jiahui Yu and Wei Lu*
aShanghai Engineering Research Center of Molecular Therapeutics and New Drug Development, School of Chemistry and
Molecular Engineering, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, P. R. China.
KEYWORDS: 2-Nitroimidazole, paclitaxel, SN-38, prodrug.
ABSTRACT: Due to the low esterase activity in human plasma, many ester and carbonate prodrugs tested in humans may be less
effective than that in preclinical animals. In this paper, PTX and SN-38 were attached to the N-1 position of 2-nitroimidazole via a
carbonate linker. Presumably, 2-aminoimidazole may help promote the intramolecular hydrolysis of the carbonate bond. The pro-
drugs exhibited a considerable stability in buffers at different pH values as well as in human plasma. Further more, a rapid reduc-
tion was exhibited in the presence of nitroreductase. An in vitro cytotoxicity assay demonstrated that hypoxic conditions could in-
crease the toxicity of prodrugs. Potentially, the compound species may form a new class of promising antitumor agents.
Paclitaxel (PTX) and camptothecin (CPT) represent the
most important anti-cancer drugs in clinical use to date and
exhibit a high anti-tumor efficacy against a wide range of tu-
the case of ester paclitaxel prodrugs in 2’-OH position, repre-
senting a common strategy for paclitaxel prodrug design.
1
7
A
large body of research studies has shown that human plasma
esterase activity is markedly different from the esterase activi-
ty in other animal species resulting in pharmacokinetic differ-
ences of these prodrugs. The efficiency of prodrugs tested in
humans may be lower than that in preclinical animals, includ-
ing mouse and rat, due to the lower esterase activity in human
1
,2
mor species. Paclitaxel is a microtubule stabilizing agent
isolated from Taxus brevifolia. Among others, paclitaxel and
its analog docetaxel are used for the treatment of solid tumors,
3
-6
including ovarian, breast, and lung cancer. Camptothecin is
a potent antitumor alkaloid isolated from Camptotheca acu-
minate and was found to target DNA topoisomerase I. Camp-
tothecin derivatives such as 10-hydroxycamptothecin, topotec-
1
8,19
plasma.
Therefore, we believe that it is necessary to intro-
duce a trigger for bond cleavage in 20-OH position of camp-
tothecin and in 2’-OH of paclitaxel in order to ensure active
drug release at the target site.
an, and irinotecan (active metabolite: SN-38) are used for the
7-10
treatment of ovarian and colorectal cancers.
Unfortunately,
these compounds exhibit significant side effects on healthy
tissues, poor solubility characteristics in aqueous media and
multidrug resistance. Ultimately, these drawbacks limit the
applicability of the drugs in clinical treatments. In an effort to
overcome these drawbacks, the development of prodrugs rep-
Hypoxia, a deficiency in the amount of oxygen reaching the
tissues, occurs in a variety of human disease states including
solid tumors. Among others, the presence of hypoxia in
20,21
solid tumors results in chemoresistance, radioresistance, angi-
ogenesis, vasculogenesis, invasiveness, metastasis, and re-
1
1-14
resents a common approach offering a site-specific release.
22,23
sistance to cell death.
Considerable effort has been put
A wide field of research studies has demonstrated that the
lactone ring in camptothecin represents a critical feature for
antitumor activity, corresponding to topoisomerase I inhibi-
tion. However, the instability of the lactone ring in the plasma
forward to develop bioreductive prodrugs which could be se-
lectively activated under hypoxic microenvironments.
Denny et al. have reported the use of prodrugs of cyclization-
2
4-27
activated aromatic mustard species involving an ortho-
1
5
28
decreases anti-tumor efficiency. Masking 20-OH by trans-
forming it to the corresponding water-insoluble alkyl ester or
carbonate has been shown to increase lactone stability in the
nitrophenyl group. Ono et al. have reported a hypoxia-
activated
pro-oligo
compounds
based
on
3-(2-
nitrophenyl)propan-1-ol with a phosphoester bond activated
1
6
29
plasma compared with their parent compounds. Some pro-
drugs conjugated with PEG in 20-OH position have been
via intramolecular cyclization of 2-aminophenol. For the
latter study, 2-nitroimidazole was demonstrated to represent an
improved bioreductive group compared to nitrobenzene.
2
4
30
demonstrated to increase the solubility in aqueous media.
However, one of the most common problems of such prodrugs
is that they have been shown to be too stable in preclinical
studies, ultimately not releasing the active compound at suffi-
ciently high concentrations. Furthermore, insufficient prodrug
conversion may reduce therapeutic efficacy due to the notion
that the active compound may never reach a therapeutically
relevant concentration. Overall, this issue may also occur in
Therefore, we designed a new class of prodrugs in N-1 posi-
tion of 2-nitroimidazole instead of the commonly used C-5
position. Here, PTX and SN-38 were conjugated to 2-(2-nitro-
1H-imidazol-1-yl)ethanol
and
3-(2-nitro-1H-imidazol-1-
yl)propan-1-ol via a carbonate bond. We further found that 2-
aminoimidazole derivatives have the ability to promote the
hydrolysis of the carbonate bond and induce the release of the
ACS Paragon Plus Environment

ACS Medicinal Chemistry Letters
Page 2 of 6
prodrugs.31 Moreover, we investigated two 2-nitroimidazole
buffers at different pH values. We then tested the compounds
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
derivatives containing ethanol and propanol in N-1 position of
2-nitroimidazole. These prodrugs were designed with signifi-
cantly increased activity at hypoxic tumor sites. One of the
most important findings of this study was that the drug release
mechanism via 2-aminoimidazole increased the drug release
efficiency (Figure 1).
in vitro in mouse and human plasma. It was known that the
much higher level of esterase in mouse plasma compared to
human plasma may accelerate the hydrolysis of the carbonate
bond. The results obtained indicate that the test compounds
degrade rapidly in mouse plasma. The half-life of 2C-SN38,
3C-SN38, 2C-PTX, and 3C-PTX in mouse plasma was de-
termined to be 47 min, 37 min, 20 min, and 55 min, respec-
tively. On the other hand, the metabolic degradation rates of
the compounds in human plasma were found to be much slow-
er. The t1/2 of 2C-SN38, 3C-SN38, 2C-PTX and 3C-PTX was
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6
h, 2 h, 2 h, 6 h. The compounds exhibited a considerable
stability in buffers at different pH values and human plasma, a
Figure 1. Proposed mechanism of prodrug activated via bio-
reduction.
finding that is consistent with the other reports found in the
12,32
literature.
Our results indicate that the carbonate bond
seems to be sufficiently stable in human plasma, a critical
feature for the future development of bioreductive prodrug
species.
The prodrugs were synthesized according to the process
outlined in Scheme 1. 2-(2-Nitro-1H-imidazol-1-yl)ethanol
and 3-(2-nitro-1H-imidazol-1-yl)propan-1-ol were activated
by addition of 4-nitrophenyl carbonochloridate and were then
reacted with PTX to afford 2C-PTX and 3C-PTX. Converse-
ly, 10-O-tert-butyldimethylsilyl-SN-38 was treated with tri-
phosgene and then reacted with 2-(2-nitro-1H-imidazol-1-
yl)ethanol and 3-(2-nitro-1H-imidazol-1-yl)propan-1-ol. The
TBS group was deprotected by addition of TBAF to afford
2C-SN38 and 3C-SN38 (Scheme 1).
Figure 2. Stability of prodrugs in different buffer. A. pH=5.0
ABS; B. pH=6.5 PBS; C. pH=7.4 PBS; D. mouse plasma; E.
human plasma.
Scheme 1. Synthetic route of prodrugs.
To determine whether the prodrug fragments provide the
desired active agent as intended, we carried out a chemical
reduction over Pd/C in the presence of H in THF at 37 °C.
2
The individual fragments were characterized by LC/MS.
Compound 3C-SN38 afforded the best results and was found
to be entirely reduced, resulting in the generation of two prod-
ucts, namely SN-38 and the 2-aminoimidazole intermediate of
The stability of the compounds was tested first as this factor
seems to be critical for biological evaluation. The compounds
were evaluated in buffer at different pH conditions (pH 5, pH
6.5 and pH 7.4) for a period of 48 h using HPLC (Figure 2).
The results indicated that the half-life (t1/2) value of 3C-SN38
and 2C-PTX in buffer at pH 5 was 40 h and 48 h, respective-
ly. The t1/2 values of 2C-SN38 and 3C-PTX were found to be
beyond 48 h. In buffer at pH 6.5, the half-life value of 2C-
SN38, 2C-PTX and 3C-PTX was 41 h, 24 h, and 24 h, respec-
tively, and the t1/2 of 3C-SN38 under these conditions was
found to be beyond 48 h. In buffer at pH 7.4, the t1/2 value of
3
C-SN38. The mass peak corresponding to the 2-
aminoimidazole intermediate could be found in the MS-ESI
spectrum (m/z = 561.02 [M+H]+) (Figure 3). Compound 2C-
SN38 was determined to be mainly reduced to a byproduct,
resulting in almost negligible release of SN-38. Presumably,
the amino group is too close to the lactone ring ultimately
causing a side reaction. On the other hand, the characterization
results using the PTX derivatives were complex. Both the 2-
aminoimidazole intermediate and PTX could be detected
while some unknown derivatives were found to be generated
2
C-PTX and 3C-PTX was 28 h and 36 h, respectively, and
the t1/2 values of 2C-SN38 and 3C-SN38 were beyond 48 h.
We concluded that these compounds were stable in different
ACS Paragon Plus Environment

Page 3 of 6
ACS Medicinal Chemistry Letters
simultaneously. We believe that the prodrugs of PTX were not
stable under the reductive conditions used here (Figure S1).
determined to be 30 min and the t1/2 values of all other com-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
pound species were within 10 min. Approximately 30% of 2C-
SN38 still remained after 1 h and only 30% of the active drug
was found to be released. Presumably, 2C-SN38 was too sta-
ble under these bioreducive conditions described. Considering
the results for 3C-SN38 and 3C-PTX, even though the pro-
drug disappeared rapidly, the generation rates of the active
drug were not as fast as the compounds reduced. Therefore,
we believe that the 2-aminointermediates, as well as some
byproducts, may have been generated. On the other hand, 2C-
PTX was determined to be the most suitable compound, with
an overall PTX conversion rate of 85% within 1 h. Likely, the
bioreductive process via nitroreductase was somewhat milder,
resulting in a reduced formation of 2C-PTX byproducts com-
pared to chemical reduction. The 2-aminoimidazole interme-
diate of 2C-PTX was not detected, which may indicate that
the drug release process of the 2-aminoimidazole intermediate
under physiological conditions was very fast.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. Percent of prodrug remaining (A) and active drug
releasing (B) in 1 h of incubation with nitroreductase in pres-
ence of NADPH.
The compounds were evaluated for their cytotoxicity char-
acteristics under normoxic and hypoxic (94% N
2 2
, 5% CO , 1%
O
2
) conditions in vitro using H460 human lung cancer cells
and HT29 human colon cancer cells. These cell lines are
known to express high levels of oxygen-insensitive reductase,
potentially activating these prodrugs via enzymatic reduction
and increasing the toxicity of the prodrugs under hypoxic con-
ditions, and were therefore used for cytotoxicity tests. The cells
were incubated in the presence of the test compounds at vari-
ous concentrations for 72 h under normoxic or hypoxic condi-
tions. The cell viability and proliferation behavior were as-
sessed by MTT. The IC50 values for the proliferation inhibition
of the tested compounds are shown listed in Table 1. The re-
sults obtained indicate that 3C-SN38 exhibited the highest
selectivity ratio of 2.03 in the H460 cell line and 2C-PTX
exhibited the highest selectivity ratio of 3.11 in the HT29 cell
line. All tested compounds featured moderate selectivity to-
wards hypoxic tumor cells. The hypoxic conditions could in-
crease the toxicity of prodrugs.
Figure 3. LC/MS charts of 3C-SN38 after chemical reduction
over Pd/C and H in THF.
2
In hypoxic tumors, the reductase level has been reported to
be much higher than in normal tissues. Presumably, this fea-
ture provides an opportunity to specifically target hypoxia. For
the further investigation of the activate mechanism of the pro-
drugs, nitroreductase (NTR) extracted from Escherichia coli
was used. The prodrugs (10 μM, 1% DMSO) dissolved in PBS
(pH 7.4) were incubated with nitroreductase (50 μg/mL) and
NADPH (1 μmol/mL) at 37 °C and the biological reduction
process was monitored by HPLC (Figure 4). The prodrugs
were found to be rapidly reduced by nitroreductase, except for
2C-SN38 which reacted slower. The t1/2 of 2C-SN38 was
Table 1. In vitro cytotoxicity assay data summary.
H460
HT29
IC50 ( μM )
IC50 ( μM )
IC50(AIR)/IC50(N
2
)
2
IC50(AIR)/IC50(N )
AIR
N
2
AIR
0.096 ± 0.048
0.014 ± 0.002
> 10
N
2
SN-38
PTX
0.050 ± 0.001
0.006 ± 0.001
0.091 ± 0.022
0.201 ± 0.022
0.047 ± 0.007
0.007 ± 0.005
0.063 ± 0.008
0.099 ± 0.012
1.06
0.86
1.44
2.03
0.119 ± 0.018
0.015 ± 0.001
9.56 ± 0.66
0.82
0.93
2
3
C-SN38
C-SN38
> 1.05
> 1.67
> 10
6.02 ± 0.52
2C-PTX
C-PTX
0.009 ± 0.001
0.021 ± 0.003
0.006 ± 0.002
0.018 ± 0.003
1.5
0.140 ± 0.041
0.290 ± 0.112
0.045 ± 0.035
0.159 ± 0.038
3.11
1.82
3
1.17
ACS Paragon Plus Environment

ACS Medicinal Chemistry Letters
Page 4 of 6
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
In summary, a new class of carbonate prodrugs on the basis
(8) Hsiang, Y. H.; Hertzberg, R.; Hecht, S.; Liu, L. F. Camp-
of 2-nitroimidazole was studied in order to release the active
drug in an efficient manner. The prodrugs exhibited a consid-
erable stability in different buffer and human plasma. The
compounds were found to be reduced rapidly in the presence
of nitroreductase. Among the studied compounds, compound
2C-PTX was determined to release 85% of the active drug,
the highest conversion rate under the studied bioreduction
conditions. Four prodrugs exhibited moderate hypoxia selec-
tivity in H460 and HT29 cell lines. On the basis of the results
obtained, we hypothesize that these prodrugs may find poten-
tial future applications as antitumor agents.
tothecin induces protein-linked DNA breaks via mammalian DNA
topoisomerase I. J. Biol. Chem. 1985, 260, 14873-14878.
(9) Wang, J. C. DNA Topoisomerases - Annual Review of Bio-
chemistry, Annu. Rev. Biochem. 1996, 65, 635-692.
(
10) Ulukan, H.; Swaan, P. W. Camptothecins: a review of
their chemotherapeutic potential. Drugs 2002, 62, 2039-2057.
11) Lis, L. G.; Smart, M. A.; Luchniak, A.; Mohanl. Gupta, J.;
(
Gurvich, V. J. Synthesis and biologicalevaluation of a biotinylat-
edpaclitaxel with an extra-long chain spacer arm. ACS Med.
Chem. Lett. 2012, 3, 745-748.
(12) Rodriguez-berna, G.; Cabañas, M. J. D.; Mangas-Sanjuán,
V.; Gonzalezalvarez, M.; Gonzalezalvarez, I.; Abasolo, I.; Simó
Schwartz, J.; Bermejo, M.; Corma, A. Semisynthesis, cytotoxic
activity, and oral availability of new lipophilic 9-substituted
camptothecin derivatives. ACS Med. Chem. Lett. 2013, 4, 651-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ASSOCIATED CONTENT
Supporting Information
6
55.
13) Wang, L.; Xie, S.; Ma, L. J.; Chen, Y.; Lu, W. 10-Boronic
The Supporting Information is available free of charge on the
ACS Publications website.
(
acid substituted camptothecin as prodrug of SN-38. Eur. J. Med.
Chem. 2016, 116, 84-89.
Experimental procedures, characterization data, and NMR spectra
PDF)
(14) Zhang, X.; Tang, K.; Wang, H.; Liu, Y.; Bao, B.; Fang, Y.;
Zhang, X.; Lu, W. Design, Synthesis, and Biological Evaluation
of New Cathepsin B-Sensitive Camptothecin Nanoparticles
Equipped with a Novel Multifuctional Linker. Bioconjugate
Chem. 2016, 27, 1267-1275.
(
AUTHOR INFORMATION
Corresponding Author
(
15) Hertzberg, R. P.; Caranafa, M. J.; Holden, K. G.; Jacas, D.
*E-mail: wlu@chem.ecnu.edu.cn. Fax: +86 21 62238771.
R.; Gallagher, M. R.; Mong, S. M.; Bartus, J.; Johnson, R. K.;
Kingsbury, W. D. Modification of the hydroxylactone ring of
camptothecin: inhibition of mammalian topoisomerase I and bio-
logical activity. J. Med. Chem. 1989, 32, 715-720.
(16) Penco, S.; Merlini, L.; Zunino, F.; Dallavalle, S. Perspec-
tives in camptothecin development. Expert Opin. Ther. Pat. 2005,
Author Contributions
‡
These authors contributed equally.
Notes
The authors declare no competing financial interest.
1
2, 837-844.
17) Zhu, Q.; Guo, Z.; Huang, N.; Wang; M.; Chu, F. Compara-
(
ABBREVIATIONS
SN-38, 7-ethyl-10-hydroxy-camptothecin;
NADPH, nicotinamide adenine dinucleotide phosphate.
tive molecular field analysis of a series of paclitaxel analogues. J.
Med. Chem., 1997, 40, 4319-4328.
(
18) Berry, L. M.; Wollenberg, L.; Zhao, Z. Esterase Activities
in the Blood, Liver and Intestine of Several Preclinical Species
and Humans. Drug Metabolism Letters, 2009, 3, 70-77.
(19) Bahar, F. G.; Ohura, K.; Ogihara, T.; Imai, T. Species Dif-
ference of Esterase Expression and Hydrolase Activity in Plasma.
J. Pharm. Sci. 2012, 101, 3979-3988.
REFERENCES
(1) Harvey, A. L. Natural products in drug discovery. Drug.
Discov. Today 2008, 13, 894-901.
(2) Newman, D. J.; Cragg, G. M. Natural products as sources of
new drugs over the last 25 years. J. Nat. Prod. 2007, 70, 461-
(
20) Wilson, W. R.; Hay, M. P. Targeting hypoxia in cancer
therapy. Nat Rev Cancer, 2011, 11, 393-410.
21) Brown, J. M. Tumor Microenvironment and the Response
to Anticancer Therapy. Cancer Biol. Ther. 2002, 1, 453-458.
22) Cairns, R. A.; Harris, I. S.; Mak, T. W. Regulation of can-
4
77.
3) Wani, M. C.; Taylor, H. L.; Wall, M.; Coggon, E. P.;
(
(
McPhail, A. T. Plant antitumor agents. VI. Isolation and structure
of taxol, a novel antileukemic and antitumor agent from Taxus
brevifolia. J. Am. Chem. Soc., 1971, 93, 2325-2327.
(
cer cell metabolism. Nature Rev. Cancer, 2011, 11, 85-95.
(23) Wang, Y.; Ohh, M. Oxygen-mediated endocytosis in can-
cer. J. Cell. Mol. Med. 2010, 14, 496-503.
(4) Mekhail, T. M.; Markman, M. Paclitaxel in cancer therapy.
Expert Opin. Pharmacother. 2002, 3, 755-766.
(
24) Baran, N.; Konopleva, M. Molecular Pathways: Hypoxia-
(5) Ferlini, C.; Ojima, I.; Distefano, M.; Gallo, D.; Riva, A.;
Morazzoni, P.; Bombardelli, E.; Mancuso S.; Scambia, G. Second
generation taxanes: from the natural framework to the challenge
of drug resistance. Curr. Med. Chem. Anti-Cancer Agents 2003, 3,
activated prodrugs in cancer therapy. Clin. Cancer Res. 2017,
DOI: 10.1158/1078-0432.CCR-16-0895.
(25) Thambi, T.; Park, J. H.; Lee, D. S. Hypoxia-responsive
nanocarriers for cancer imaging and therapy: recent approaches
and future perspectives. Chem. Commun. 2016, 52, 8492-8500.
(26) Wigerup, C.; Påhlman, S.; Bexell, D. Therapeutic targeting
of hypoxia and hypoxia-inducible factors in cancer. Pharmacol.
Ther. 2016, 164, 152–169.
1
33-138.
6) Nabholtz, J. M.; Vannetzel, J. M.; Llory, J. F.; Bouffette, P.
(
Advances in the use of taxanes in the adjuvant therapy of breast
cancer. Clin. Breast Cancer 2003, 4, 187-192.
(7) Wall, M. E.; Wani, M. C.; Cook, C. E.; Palmer, K. H.;
(27) Damen, E. W. P; Nevalainen, T. J.; Toine J. M. van den
McPhail, A. T.; Sim, G. A. Plant Antitumor Agents. I. The Isola-
tion and Structure of Camptothecin, a Novel Alkaloidal Leukemia
and Tumor Inhibitor from Camptotheca acuminate. J. Am. Chem.
Soc. 1966, 88, 3888-3890.
Bergh; Franciscus M. H. de Groot; Scheeren, H. W. Cheminform
abstract: synthesis of novel paclitaxel prodrugs designed for bio-
ACS Paragon Plus Environment

Page 5 of 6
ACS Medicinal Chemistry Letters
reductive activation in hypoxic tumor tissue. Bioorg. Med. Chem.
2002, 10, 71-77.
(28) Atwell, G. J.; Sykes, B. M.; O'Connor, C.J.; Denny, W. A.
Relationships between structure and kinetics of cyclization of 2-
aminoaryl amides: potential prodrugs of cyclization-activated
aromatic mustards. J. Med. Chem. 1994, 37, 371-380.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
29) Saneyoshi, H.; Iketani, K.; Kondo, K.; Saneyoshi, T.;
Okamoto, I.; Ono, A. Synthesis and Characterization of Cell-
Permeable Oligonucleotides Bearing Reduction-Activated Pro-
tecting Groups on the Internucleotide Linkages. Bioconjugate
Chem. 2016, 27, 2149-2156.
(30) O’Connor, L. J.; Cazares-Körner, C.; Saha, J.; Evans, C. N.
G.; Stratford, M. R. L.; Hammond, E. M.; Conway, S. J. Design,
synthesis and evaluation of molecularly targeted hypoxia-
activated prodrugs. Nat. Protoc. 2016, 11, 781-794.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(31) Hanaya, K.; Yoshioka, S.; Ariyasu, S.; Aoki, S.; Shoji, M.;
Sugai, T. Development of a novel sulfonate ester-based prodrug
strategy. Bioorg. Med. Chem. Lett. 2016, 26, 545-550.
(33) He, X.; Lu, W.; Jiang, X.; Cai, J.; Zhang, X.; Ding, J. Syn-
thesis and biological evaluation of bis and monocarbonate pro-
drugs of 10-hydroxycamptothecins. Bioorg. Med. Chem. 2004, 12,
4
003–4008.
Insert Table of Contents artwork here
ACS Paragon Plus Environment

ACS Medicinal Chemistry Letters
Page 6 of 6
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
In this paper, we uncovered that 2-aminoimidazole may form an intramolecular promotion to hydrolysis of
carbonate prodrugs of PTX and SN-38.
1
89x41mm (96 x 96 DPI)
ACS Paragon Plus Environment